An OFDM (orthogonal frequency division multiplexing) transmission system is known as a transmission system suitable for digital transmission using a terrestrial wave due to its tolerance to multipath interference.
When transmitting using the OFDM transmission system, the channels are arranged with a predetermined frequency intervals (guard bands) between them, for example, as shown in FIG. 1, so as to prevent the interference between adjacent channels. In FIG. 1, an example is shown where the provision (setting) is made guard bands between the channels when the center frequency of a channel 1 (ch1) is f1, a center frequency of channel 2 (ch2) is f2, and a center frequency of channel 3 (ch3) is f3.
FIG. 2 is a block diagram of the configuration of a transmitter 1 transmitting a signal provided with guard bands illustrated in FIG. 1. The transmitter 1 shown in FIG. 2 shows the configuration in the case of transmitting three channels worth, ch1 to ch3, of signals.
An information sequence 1 of ch1 input to a mapping unit 2-1 is assigned to modulated signal points of a predetermined coding scheme, for instance, QPSK (quadrature phase shift keying) and 16 QAM (quadrature amplitude modulation), and is output to an IFFT (Inverse Fast Fourier Transform) processor 3-1. The IFFT processor 3-1 performs an inverse Fourier transform on the input modulated signal points and outputs the result to a guard interval inserter 4-1. The guard interval inserter 4-1 inserts a guard interval to the input signal and outputs the result to a quadrature modulator 5-1. The quadrature modulator 5-1 uses a carrier wave of a frequency fIF generated at an oscillator 6-1 to quadrature-modulate the input signal and outputs the result to a frequency converter 7-1. The frequency converter 7-1 multiplies a carrier wave with a frequency f1+fIF which is the sum of the center frequency f1 of the channel 1 and the intermediate frequency f1F generated at the oscillator 8-1 with the input signal, converts the result to an RF band signal of the center frequency f1 of the channel 1, and makes the result be transmitted from an antenna 9-1.
An information sequence 2 of ch2 is input to a mapping unit 2-2, while an information sequence 3 of ch3 is input to mapping unit 2-3. These are processed to the same processing as the above-mentioned information sequence 1 by corresponding devices, whereby they are made into an RF band signal of the center frequency f2 and an RF band signal of the center frequency f3.
The resultant three channels' worth of signals are, as shown in FIG. 1, provided with guard bands between the adjacent channels, and thus, when detecting a desired channel at a receiving side, the detection of the desired channel is free from the affection of its adjoining channels.
As described above, the guard band is provided to prevent the desired channel from being affected by its adjoining channels, but, the provision of the guard band results in a low efficiency of frequency utilization and it is not desirable from the viewpoint of the effective use of frequency. However, if the guard band is not provided or the distance is made narrower, it suffers from the disadvantages that the adversely affection from the adjoining channels may occur and the S/N at the time of reception may deteriorates.